This study proposes a cross layer volume fracturing technology for horizontal wells of continental shale oil and gas, including evaluation of the compressibility of continental shale oil and gas reservoirs, optimization of fracture parameters with EUR as the target, optimization of fracturing construction parameters based on single cluster fracture simulation, optimization of entire process cross layer fracturing technology based on improving the far well fracture height, integrated fracturing fluid system and performance evaluation of imbibition and oil displacement, and optimization method of post pressure sealing system based on imbibition mechanism, In order to improve the fracturing level and exploration and development benefits of continental shale oil and gas.

1.Evaluation of Compressibility of Continental Shale Oil and Gas Reservoirs

The terrestrial lake basin is relatively close to the source of matter and is significantly influenced by factors such as structure and climate. The genetic mechanism of terrestrial fine-grained sedimentary rocks is not only a single "slow suspended sedimentation in a quiet water environment", but also a transport factor such as turbidity current, density current, fluid sedimentary current, and storm action. Taking the continental shale and Fuling marine shale in the Yuanba and Fuxing regions of the Sichuan Basin as examples, the compressibility parameters of continental sedimentary shale and marine shale were compared. The results showed that compared with marine shale, the total organic carbon content, gas content, brittle mineral content, elastic modulus, and natural fracture development degree of continental shale were relatively low, while clay content, Poisson's ratio, and horizontal stress difference were relatively high. Overall, the compressibility of continental shale is relatively low.

2.Optimization of Interlayer Fracturing Parameters and Processes

Optimization of fracture parameters for interlayer fracturing with EUR as the target. Shale oil reservoirs have strong vertical heterogeneity, with thin interbeds or interlayers, significant stress shielding, and well-developed interlayers, which makes it very difficult to expand the fracture height and greatly affects the volume of fracturing transformation. Therefore, in order to achieve good fracturing results, it is generally necessary to adopt the process mode of interlayer fracturing. Due to the difficulty in obtaining EUR, the cumulative oil production over 10 years is used instead of EUR for analysis. After optimizing the fracture parameters of the interlayer fracturing with EUR as the goal, it is necessary to optimize the fracturing process parameters with the optimized combination of fracture parameters as the goal.

Optimization of interlayer fracturing process parameters based on single cluster fracture simulation. Due to the complexity of continental shale oil extraction, in order to improve mining efficiency, a few segments and multiple clusters fracturing mode is usually adopted. How to carry out the graded crack initiation and extension before pitching, and prevent all clusters from opening to varying degrees before pitching, is an important prerequisite for improving the temporary plugging ball fracturing effect. Based on physical and numerical simulation research, several process methods such as low displacement and long-term injection, optimized perforation methods, and non-uniform acid distribution are proposed to achieve staged fracturing.

Optimization of the entire process through layer fracturing process based on improving the height of far wellbore fractures. At present, the main approach is to use a reverse fracturing mode based on high viscosity gel pre loading, combined with a strategy of quickly increasing displacement, to quickly accumulate enough high pressure in the wellbore. A multi-level temporary plugging mode has also been proposed, but it can only achieve temporary plugging in the fracture once. Therefore, a strategy of temporary plugging in the primary or secondary seam is proposed. After the implementation of secondary temporary plugging, due to the agglomeration effect of the temporary plugging agent, it will hold pressure at a certain position in the direction of the seam length, causing an increase in the net pressure inside the seam and further expanding the seam height at the temporary plugging location.

3.Imbibition and oil displacement integrated fracturing fluid and its performance evaluation

Fracturing fluid with the integrated function of imbibition and oil displacement is an inevitable choice. By adding an imbibition agent to the fracturing fluid, the hydration imbibition effect is further promoted. At the same time, adding oil displacement agents to the fracturing fluid can further reduce the flow resistance of oil and gas during the post fracturing production process. By selecting anionic surfactants with special structures, the tight arrangement of surfactant molecules on the solution surface is achieved, reducing surface tension. Adding a certain amount of cationic surfactant increases the tightness of the surface molecule adsorption membrane, further synergistically reducing surface tension. Adding amphoteric surfactants to the formula to enhance low interfacial tension and peel off performance of heavy components in oil reservoirs. Introducing non ionic surfactants and utilizing their good permeability to further enhance interfacial oil detachment.

4.Optimization of post pressure sealing system based on imbibition

In theory, the optimal shut-in time can be determined based on the standard of a continuous 3d decline rate of wellhead pressure approaching 0, but this may take a long time and on-site conditions do not allow it, so a relatively compromise solution is needed. Because when the well is initially blocked, the daily pressure drop rate is relatively large, such as greater than 2 MPa/d. But as the lockdown time prolongs, the decrease rate of wellhead pressure gradually slows down, sometimes even below 0.1 MPa/d. Especially when the pressure drops below a certain critical value, the rate of pressure drop also significantly decreases. Sometimes, the pressure drops by 0.1 MPa, which may take more than 10 days or even more than a month. Therefore, a compromise of 0.1 MPa/d can be taken as the optimization basis for the shut in time.

5.Field Test

The in-situ pilot test of the volume fracturing technology for horizontal wells of continental shale oil and gas was conducted in wells A and B of the shale oil and gas horizontal wells. Among them, well A carried out fracturing parameter optimization with the goal of EUR=30000 cubic meters, adopting the main idea of "dense cutting section cluster division, complex fracture creation, and imbibition production increase". A total of 88000 cubic meters of fracturing fluid were pumped, 8600 cubic meters of proppant and 7.5 tons of temporary plugging agent were used. The integrated fracturing fluid system of imbibition and oil displacement was used, and the well was sealed for 15 days according to pressure changes after fracturing. The inverted fracture height after fracturing was 22-26 meters, covering all the production layers (16-23 meters), Realized interlayer fracturing. Well B carried out fracturing parameter optimization with EUR as the goal, and a total of 23 stages of fracturing construction were carried out. 51900 cubic meters of fracturing fluid were pumped, 3250 cubic meters of proppant were used, and 2.4 tons of temporary plugging agent were used. After fracturing, the well was sealed for 20 days according to the pressure changes. After fracturing, the inversion of the fracture height was 17-20 meters, covering the main production layer (15-19 meters), achieving good transformation results.

6.Conclusions and Recommendations

1).The study proposed a volume fracturing technology for horizontal wells of continental shale oil and gas, which mainly includes reservoir compressibility evaluation, optimization of fracturing parameters, optimization of fracturing process parameters, integrated fracturing fluid for imbibition and oil displacement, and optimization of well sealing time.

2).The on-site test results indicate that the interlayer volume fracturing technology for continental shale oil and gas horizontal wells has good adaptability, pertinence, and operability, and can effectively cover the fracture height of multi layered continental shale oil and gas reservoirs.

3).To achieve true interlayer volume fracturing, it is necessary to strengthen the integration of geology engineering and drilling and fracturing.